This invention relates to an improvement in glycol dehydration apparatus used for drying natural gas. More particularly, the invention concerns the use of a shell-and-tube condenser on the water vapor vent of a glycol reboiler to remove volatile carbon compounds from the vent gas.
Processing of natural gas flowing from the wellhead is required to provide a uniformly lean, sweet and dry gas for industrial and consumer use. Processing facilities are usually located in or near natural gas fields and range in size from small wellhead installations to large plants.
Gas processing involves the use of a number of different processes, each designed to remove a different impurity. Impurities range from acid gases such as carbon dioxide, hydrogen sulfide, carbonyl sulfide and other sulfur compounds to water and higher homologues of methane such as ethane, propane and butane. The higher alkanes are separated out because of their greater value.
Water vapor, which is usually present in gas produced from underground formations, must be removed either by absorption by hydroscopic liquids or adsorption on beds of molecular sieves or activated solid dessicants. The most widely used water removal systems in the industry are those based on water adsorption by ethylene glycol, diethylene glycol or triethylene glycol. Diethylene glycol is the most widely used absorbent. The advantages of relatively low cost, ease of regeneration and operation, minimal losses of drying agent during operation, and thousands of glycol dehydration units in the field insure that glycol dehydration will continue to be the predominant means of water removal from natural gas in the future.
Glycol dehydration units consist of a contacting or absorber tower, a glycol reboiler to regenerate the glycol, a three-phase gas, glycol and condensate separator and a pressure reduction pump to reduce the pressure of the wet glycol before it moves to the glycol reboiler for regeneration. In the contactor tower, the wet gas stream comes into contact with the countercurrent flow of glycol. The pressure reduction pump reduces pressure of the wet glycol before it passes to the glycol reboiler or regenerator, where the water is distilled from the glycol. Most units then employ a three-phase gas and glycol condensate separator to recover some of the hydrocarbons from the glycol prior to drying the glycol in the glycol reboiler.
Recently, the natural gas industry has become aware that the water vapor vented from the stripping still of the glycol reboiler is rich in volatile carbon compounds, including toxics such as benzene, toluene, ethylbenzene and xylene, making the water vapor vent gas an air pollution source that must be curtailed. One solution to this pollution problem has been the use of forced draft condensers attached to the water vapor vent line for the purpose of condensing the hydrocarbons in the vent gas. Unfortunately, forced draft condensers are costly and have moving parts to create reliability problems. Electrical power is required and the condensers are sensitive to ambient air temperature. If the air temperature is hot, as frequently occurs in southern United States gas fields, the forced draft condensers will be unable to adequately condense the volatile carbon compounds in the water vapor vent gas.
A second solution that is being tried by industry as direct incineration. However, this solution is fraught with substantial technical and cost problems, not the least of which is heating large quantities of water. Natural draft condensers have been tried but are not very effective. Furthermore, they are large and relatively costly.